The present invention relates to a device for detecting a residual capacity of a secondary battery and a device for controlling a secondary battery.
A secondary battery is widely used as an electric energy source for an electric car or cordless equipment such as OA equipment, AV equipment, electric power tools, toys and communication equipment. As the secondary battery is more widely used, the solution for various technical problems about the secondary battery has been more strongly requested. Of these technical problems, it is likely that the technique for detecting and indicating a residual capacity in the battery is considered as a relatively slight problem by the manufacturers. However, this technique is a significant problem for the users. This is because the accurate grasp of an available time of the equipment makes it possible to use such equipment more conveniently and favorably. In particular, this problem is quite significant to an electric car.
A currently proposed system for detecting and indicating a residual capacity of a battery mainly uses a capacity meter for detecting a battery voltage, an open-circuit voltage, a discharge quantity, and a relative density of electrolytic solution ("Report for Searching and Studying Capacity Meter for Residual Capacity for Electric Car", pages 8 and 19, 1988, edited by Japan Electric Car Society Foundation).
The U.S. Pat. No. 4,952,862 discloses a device for estimating a battery residual capacity by deriving a battery discharge adaptive condition from a battery temperature, a battery voltage and an I/O current. As another prior art, the U.S. Pat. No. 4,497,881 discloses an indicator for a residual capacity which is arranged to observe a color of a compound in the battery through a battery window for detecting a residual capacity of the battery.
The aforementioned systems arranged to use a capacity meter for detecting a residual capacity of a battery has been theretofore proposed. These systems, however, have their inherent problems. Hence, they are not practically usable. The system for detecting a battery voltage has some problems. First, the battery voltage changes according to the quantity of discharge current of the battery. Second, the voltage in an open-circuit state is greatly different from the voltage in a closed-circuit state. Third, the system for detecting battery voltage is difficult to apply to a battery having excellent discharge voltage flatness. The system for detecting a voltage in an open-circuit state uses such a characteristic of a lead-acid battery as changing an open-circuit voltage of a lead-acid battery according to the discharge amount. However, the system for detecting a voltage in an open-circuit state is not applied to a battery having excellent discharge voltage flatness such as a nickel-cadmium battery, a nickel-hydrogen battery, a nickel-metal hydride, a nickel-zinc battery, and a zinc-air battery. Further, the change of a voltage when a closed circuit is exchanged to an open circuit depends on the quantity of discharge current in an open circuit. This dependency may give rise to an inaccurate measured value when the battery is in use.
The system for detecting a quantity of discharge current is not arranged to consider adverse effects such as self-discharge, a discharge temperature, and regenerative charge. As another problem, the reference discharge capacity of a battery is reduced as the charge and discharge cycle of the battery is progressing. The system for detecting a relative density of electrolytic solution is based on such a characteristic of a lead-acid battery as reducing a sulfuric acid concentration in electrolytic solution in the lead-acid battery and uses a relative density meter, for example, for detecting the sulfuric acid concentration. This system, therefore, may not apply to a sealed lead-acid battery having no free electrolytic solution or any other battery except the lead-acid battery.